Resonant fiber optic gyroscopes (RFOGs) have greater signal to noise sensitivity for rotation sensing (“rotation sensitivity”) for a given diameter than laser ring gyroscopes and interferometer fiber optic gyroscopes. Thus, RFOGs are desirable because a given level of signal-to-noise sensitivity to rotation rate can be achieved with smaller RFOGs, reducing the size of inertial navigation systems incorporating RFOGs. RFOGs, however, are susceptible to bias error because of imperfections in RFOG optical components, e.g. lasers, and RFOG control and signal processing errors.
One source of bias error arises due to variations in a differential power level of the optical signals propagating in an optical resonator in each of the clockwise and counter-clockwise paths. Because a conventional optical resonator coil of an RFOG is made from glass fiber, if the differential power level varies, then—due to the Kerr effect—a difference in index of refraction is created in the two propagation paths. This creates undesirable bias error in the RFOG measurements.
To address this problem, the power levels of the optical signals are controlled. Conventionally, this has been done by extracting a portion of each of the clockwise and the counter-clockwise optical signals emitted from the optical resonator coil by coupling the light into polarizing maintaining (PM) optical fibers prior to detection. The portions may be extracted by optical circulators after coupling into the PM optical fibers, each of which is coupled to a power detector. The PM optical fibers are also used to couple the clockwise and counter clockwise optical signals into the optical resonator. The extracted portions and inputted signals also pass through polarizers.
Rotational misalignments of the PM fiber cause each portion to be launched into and propagate through the PM optical fiber as two components having different polarizations. The polarization of the two components becomes out of phase before the components are incident on the circulator, which contains a polarizer or polarization sensitive elements. Because the polarizer attenuates one of the polarization components, the portion incident upon each power detector is no longer representative of the power level of the corresponding optical signal propagating in the optical resonator. Thus, the measurements of the power detectors are erroneous, due to “polarization misalignment” and subsequent polarization filtering. As a result, bias error from variations in the power levels of the optical signals propagating in the optical resonator is not satisfactorily diminished, and may even be increased due to erroneous indications that the power has changed in resonator (and feedback based on erroneous indications). In addition, coupling of the light into the PM optical fiber may vary, especially over temperature, due to tight spatial alignment tolerances (“spatial misalignment”). This would also cause erroneous indication that optical power circulating in the optical resonator has changed, when it has not.